Volcanic Aerosols Tamped Down Recent Surface Warming

The last decade was the hottest on record. And the data make clear the planet is still warming, despite deniers’ disinformation to the contrary. But a new study does explain one reason surface temperatures did not rise quite as much as scientists expected in the past decade – JR.

In the search for clues as to why Earth did not warm as much as scientists expected between 2000 and 2010, researchers have discovered the answer is hiding in plain sight. The study, led by a scientist from NOAA’s Cooperative Institute for Research in Environmental Sciences (CIRES), showed that dozens of volcanoes spewing sulfur dioxide have tempered the warming.

The findings essentially shift the focus away from Asia, including India and China, two countries that are estimated to have increased their industrial sulfur dioxide emissions by about 60 percent from 2000 to 2010 through coal burning, said lead author Ryan Neely, a CIRES scientist working at NOAA’s Earth System Research Laboratory. Small amounts of sulfur dioxide emissions from Earth’s surface eventually rise 12 to 20 miles into the stratospheric aerosol layer of the atmosphere, where chemical reactions create sulfuric acid and water particles that reflect sunlight back to space, cooling the planet.

Neely said previous observations suggest that increases in stratospheric aerosols since 2000 have counterbalanced as much as 25 percent of the warming scientists attribute to human greenhouse gas emissions. “This new study indicates it is emissions from small to moderate volcanoes that have been slowing the warming of the planet,” said Neely.

A paper on the subject was published online in Geophysical Research Letters, a publication of the American Geophysical Union. Co-authors include Professors Brian Toon and Jeffrey Thayer from CU-Boulder; Susan Solomon, a former NOAA scientist now at the Massachusetts Institute of Technology; Jean Paul Vernier from NASA’s Langley Research Center in Hampton, Va.; Christine Alvarez, Karen Rosenlof and John Daniel from NOAA; and Jason English, Michael Mills and Charles Bardeen from the National Center for Atmospheric Research in Boulder.

The new study relies on long-term measurements of changes in the stratospheric aerosol layer’s “optical depth,” which is a measure of transparency, said Neely. Since 2000, the optical depth in the stratospheric aerosol layer has increased by about 4 to 7 percent, meaning it is slightly more opaque now than in previous years.

“The biggest implication here is that scientists need to pay more attention to small and moderate volcanic eruptions when trying to understand changes in Earth’s climate,” said Toon of CU-Boulder’s Department of Atmospheric and Oceanic Sciences. Overall these eruptions are not going to counter the human caused greenhouse warming, he said. “Emissions of volcanic gases go up and down, helping to cool or heat the planet, while greenhouse gas emissions from human activity just continue to go up.”

The key to the new results was the combined use of two sophisticated computer models, including the Whole Atmosphere Community Climate Model, or WACCM, Version 3, developed by NCAR and which is widely used around the world by scientists to study the atmosphere. The team coupled WACCM with a second model, the Community Aerosol and Radiation Model for Atmosphere, or CARMA, which allows researchers to calculate properties of specific aerosols and which has been under development by a team led by Toon for the past several decades.

Neely said the team used the Janus supercomputer on (the CU) campus to conduct seven computer “runs,” each simulating 10 years of atmospheric activity tied to both coal-burning activities in Asia and to emissions by volcanoes around the world. Each run took about a week of computer time using 192 processors, allowing the team to separate coal-burning pollution in Asia from aerosol contributions from moderate, global volcanic eruptions. The project would have taken a single computer processor roughly 25 years to complete, said Neely.

The scientists said 10-year climate data sets like the one gathered for the new study are not long enough to determine climate change trends. “This paper addresses a question of immediate relevance to our understanding of the human impact on climate,” said Neely. “It should interest those examining the sources of decadal climate variability, the global impact of local pollution and the role of volcanoes.”

While small and moderate volcanoes mask some of the human-caused warming of the planet, larger volcanoes can have a much bigger effect, said Toon. When Mount Pinatubo in the Philippines erupted in 1991, it emitted millions of tons of sulfur dioxide into the atmosphere that cooled the Earth slightly for the next several years.

The research for the new study was funded in part through a NOAA/ ESRL-CIRES Graduate Fellowship to Neely. The National Science Foundation and NASA also provided funding for the research project. The Janus supercomputer is supported by NSF and CU-Boulder and is a joint effort of CU-Boulder, CU Denver and NCAR.

NASA GISS forcing data doesn’t show much evidence of a significant cooling effect from stratospheric aerosols. There has been a decline in the growth rate of anthropogenic greenhouse gases combined with predominantly La Niña conditions in the Pacific in recent years, and that seems to be sufficient to explain the evolution of global temperature as explained by Tamino. As he says, regarding the supposed ‘pause’ in global temperature rise, “there’s nothing to explain“.

Assuming this result holds up to further study (always a pertinent caveat when talking about “new” information), which I suspect it will, then I think this tells us:

The environment is naturally twitchy, even without getting any “help” from humanity in doing interesting things.

We are continuing to treat the atmosphere like an infinitely capacious sewer, which only ensures that when we hit a lull in volcano activity and we see China and India get increasingly serious about reducing their sulfur emissions, we’re in for one heck of wild ride. Right now, anthro aerosols are masking a lot of warming from CO2 (and CH4), so reducing that parasol effect by even a little will result in a big boost in warming.

I suppose, as we melt out the Ice covered regions, we should expect a spike in some volcanic activity as volcanoes that were close or overdue to blow and had lots of ice/snow on top of them get that melted.

I remember reading an article on Volcanoes in Iceland (I believe) where they had one that was big, was way overdue to blow and had thousands and thousands of feet of snow on top (which presumably will be melting away).

Jim I’m right there with you as well. We had these last 4 years of are you kidding me political setback articles which seem to have slowed up somewhat (or I’ve gotten numb to them) but now we have a new tranche of “Well its looking like the science is much worse than we thought 5 years ago when we told you things were much worse than we thought 5 years before….”.

The Arctic is going to be the tough one to watch for me over this decade. We’ve got to gird ourselves for more of this as things will probably continue to happen faster and worse than scientists keep predicting – and be ready to lend our voices and actions to turn what we can around when the opportunities arrise.

Would American politicians take the science more seriously if the hottest decade with severe weather and other climate impacts were more extreme? Obama’s ‘blend’ of energy sources needed to move America forward would likely be less popular.

Thanks for that link, Aldous, I was skeptical about the Amazon turning into savannah this century myself.

Boreal forests are a much bigger problem. Trees grow slowly in the frozen north, and sequester far more carbon than those around the equator, on a gross and per hectare basis. As temperatures rise, we will see more heated permafrost and forest fires- phenomena that are already occurring.

I may be wrong but I think the point of this publication is that, in fact, volcanic SO2 emissions are what have been driving the increased optical depth of the atmosphere’s stratospheric aerosol layer, *not* anthro aerosols.

So, if the results hold up, this is good news because volcanic emissions will always be present (though perhaps return to average levels), whereas anthro aerosols will go to near zero when (hopefully) we shut down the major sources.

Looking at NASA GISS forcings data, I see “reflective tropospheric aerosols” (i.e. the stuff from coal power plants) as separate from the stratospheric aerosol effect (the volcanic stuff talked about in this article).

Your America, Mike, the America of human decency and Enlightenment morality, has lost its way, because the other America, the America of the greedy plutocrats and the crudely materialistic and plain dumb lumpen masses, saturated in hyper-jingoistic chauvinism, corn syrup fructose and delusions of cultural superiority to the rest of humanity, has totally usurped all the institutions and functions of power in your country. This Rightwing triumph is replicated throughout the Anglosphere and, to an increasing degree, in the West as a whole, and it is the root cause of the looming catastrophe. Rule by the Right, with all humane and compassionate alternatives extirpated, is a certain recipe for disaster, (as we are seeing), social, economic, ecological and spiritual.